This invention relates to a novel tank arrangement, and is particularly directed to a tank for processing parts in chemical milling operations, containing a combination of features and elements for efficiently circulating the processing fluid, such as a water base maskant to be applied to parts prior to chemical milling thereof.
Particularly in the aircraft industry, chemical milling employing chemical etchants such as acids and bases are applied to structural components for shaping such components by subjecting them to a controlled deep etching. In such process, generally a maskant is first applied to the parts as a coating to protect the maskant covered areas of the parts from being etched when the parts are subjected to treatment in the etchant solution to etch preselected areas of the parts from which maskant has been removed.
A water base mask system has recently been developed by DeSoto Company of Des Plaines, Ill., which includes a precoat, maskant and topcoat. The liquid maskant material is a water base material in the form of a liquid latex type composition. Such maskant material has several major shortcomings. It retains air bubbles and congeals at the fluid-air-interface unless it is agitated, but excessive mechanical energy applied to the maskant material causes rapid degradation from a liquid latex type coating to a useless, hard latex rubber. Thus, the liquid maskant material is a shear sensitive material and if agitated too violently, it breaks down and forms undesirable particulate matter.
In an initial tank design to overcome the above shortcomings, use of a positive displacement pump, e.g. of the diaphragm type, produced insufficient fluid flow rates to promote adequate agitation of the maskant. The net result was entrained air bubbles, rapid maskant material degradation within the diaphragm pump, and undesirable settling of maskant solids.
Various fluid circulation and agitating devices have been developed in the prior art for use with various fluids. Thus U.S. Pat. No. 2,791,406 to Bates discloses agitating devices for a rectangular tank, employing agitators which discharge through elbows to provide directed flow. The device disclosed in the patent creates a high velocity stream across the fluid surface. If the patent device were employed for circulating water base maskant of the type noted above for use in chemical milling operations, eddy currents would tend to form along the walls, and would result in promoting mask skinning in low velocity areas.
U.S. Pat. No. 3,737,288 to Hochman discloses a draft tube-type reactor, utilizing a close clearance high speed agitator to draw materials into the tube. A semicircular flow deflector is mounted at the discharge end of the draft tube reactor, thereby directing the flow in a turbulent stream along the outer wall of the draft tube.
U.S. Pat. No. 4,278,460 to Chrisman, et al, discloses a method and apparatus for mixing molten glass employing spiral stirrers in draft tubes to blend and homogenize molten additives into a high viscosity host glass composition as it flows through a channel. The blending end homogenizing is performed by the rotating spiral stirrer operating within the draft tube, simultaneously mixing and pumping the molten mixture therethrough. A dam is positioned atop the draft tube or mixing block to stabilize the mixing block exit flow.
One object of the present invention is the design of a tank adapted to contain a liquid, particularly a water base maskant liquid for use in chemical milling operations, to provide high pumping rates for sufficient fluid flow and agitation to remove air bubbles and prevent congealing while minimizing mechanical energy transfer to the fluid.
Another object is the provision of a tank particularly designed for containment, agitation and circulation of chemical milling solutions, particularly a water base chemical milling maskant.
Other objects and advantages of the invention will appear hereinafter.